1. Field of the Invention (Technical Field)
The present invention relates generally to high power lasers and in particular to integrated high power diode pumped laser devices.
2. Description of Related Art
There are generally two types of semiconductor lasers. Edge-emitting lasers propagate laser light parallel to the wafer surface of the semiconductor chip and reflect the light out at a cleaved edge. Surface-emitting lasers propagate light in the direction perpendicular to the semiconductor wafer surface. Edge-emitting lasers are the most widely used form of semiconductor laser. They are capable of obtaining an output with high output power, although they require correction for poor beam quality.
A laser is typically composed of an active laser medium and a resonant optical cavity. The active laser medium, also referred to as a gain medium, is a material of a specific purity, size and shape which amplifies the beam. The gain medium must be pumped by an external energy source, such as another laser. The resonant cavity contains a coherent beam of light between reflective surfaces such that each photon passes through the gain medium multiple times before being emitted from the output aperture or lost to diffraction or absorption.
Rare-earth metals are predominately used to dope a crystal in forming a laser gain medium. Example doped crystal useable in gain media include ytterbium-doped yttrium aluminum garnet (Yb:YAG). Ytterbium has been attractive for laser crystals because of its small quantum defect allowing for very high power efficiencies, as well as large gain bandwidth. Other rare-earth metals used in doped crystal include neodymium and erbium.
The resonate cavity, or optical cavity, includes an arrangement of mirrors, typically two facing flat mirrors or spherical mirrors. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss. As more wave energy enters the cavity, it combines with and reinforces the standing wave and increases its intensity.
Laser diodes can be used to pump laser gain media, referred to as a pump laser. The output of a typical semiconductor laser diode diverges almost immediately on exiting the aperture, at an angle that may be as high as 50 degrees. A perfectly collimated beam cannot be created because of diffraction. Thus, typically a divergent beam is transformed into a collimated beam by means of a lens.
High power lasers have been produced using edge-emitting lasers. Edge-emitting lasers are arranged into compact bars, and an array of bars can be arranged into a stack. These small self-contained high power stacks are available as a packaged laser device. The high power stacks offer efficient cooling, and high output power per bar. Packages consisting of high power stacks of laser diode bars are particularly useful for applications such as laser welding, heat-treating, brazing, and laser sintering.
Examples of packaged high power stacks of laser diode bars are disclosed in U.S. Pat. No. 5,898,211 to Marshall et al., entitled “Laser Diode Package with Heat Sink.” Marshall discloses arrangements where a plurality of laser diode bars are grouped together to form an array. FIG. 7 illustrates an example embodiment of an edge emitted laser diode bar, referred to as “laser diode package” 10. The package 10 includes an associated heat sink 14. FIG. 8 illustrates an example embodiment of an assembly having an array of laser diode bars. A laser diode array 100 includes a plurality of laser diode bars 10 arranged in substantially parallel configuration. A heat sink 14 of each of the laser diode bars 10 is bonded to a backplane 110 which is further attached to a thermal reservoir 120. The laser diode packaging arrangement of Marshall is self-contained and has its own cooling system.
An example laser having a laser diode pumping a laser gain medium is disclosed in U.S. Pat. No. 4,847,851 to Dixon, entitled “Butt-Coupled Single Transverse Mode Diode Pumped Laser.” Dixon discloses a solid state laser that is optically pumped by a semiconductor diode. The output facet of the pumping diode is butt-coupled to the input facet of the pumping diode of the laser gain medium. Dixon discloses embodiments wherein the pumping diode and the input facet of the laser gain medium may be bonded together by a suitable optical adhesive, resulting in a device of reduced size.
In a preferred embodiment, Dixon discloses diode pump fixedly secured to a mounting member of copper and in turn to a mounting block. The laser gain medium is secured to the mounting block. The mounting blocks are maintained at a desired temperature by a thermal electric cooler. A hot face of the cooler is mounted to a heat sink.
U.S. Pat. No. 5,651,021 to Richard et al., entitled “Diode Pumped Slab Laser,” discloses an alternative diode pumped slab laser, including embodiments for 3-bar stack diodes. Because pumped operation results in astigmatic focusing, Richard et al. discloses that improved matching between pump and mode at high inputs can be achieved by using a concave totally reflecting mirror.
U.S. Pat. No. 6,157,663 to Wu, et al., entitled “Laser with Optimized Coupling of Pump Light to a Gain Medium in a Side-Pumped Geometry,” discloses, as an alternative to an end-pumped configuration, a solid-state laser that includes a high-absorption coefficient solid-state gain medium that is side pumped with a semiconductor laser diode array. Wu et al. discloses a preferred arrangement for side pumping the gain medium. Wu et al. discloses spacing between the pump laser and the gain medium that is greater than the conventional butt coupling configuration. The spacing is chosen to optimize coupling into a TEM00 mode having a fixed position within the gain medium. In other words, the spacing is selected to maximize output power from the particular mode. Wu et al. discloses an embodiment wherein the semiconductor laser and the gain medium may be mounted on the same block and cooled together, e.g., copper block.
It has been proposed to use the high power stack packages to pump a slab laser (Comaskey et al., “High Average Power Diode Pumped Slab Laser,” IEEE J. Quant Elec., Vol. 28, No. 4, April 1992). In particular, Comaskey describes a diode pumped slab laser having edge emitting diode bars face pumping one side of a YAG slab. The high power stacks are used to face pump the planar waveguide in order to obtain a laser of high average power. A cylindrical glass rod concentrates each line source emission into the center of the slab.
However, packaged high power lasers having high power stacks of edge emitting diode bars have several disadvantages. Light output from edge emitters is highly astigmatic. The light becomes highly divergent upon emission from the diode surface. Lenses are required for reduced divergence. Also, emitted light from edge emitting diodes is elliptical in shape. In order to obtain a circular shape, beam shaping optics is required. Furthermore, the high power stacks and laser waveguide, each being their own separate package each have their own independent cooling systems. FIG. 9 shows high power stacks of edge emitting diode bars with the laser gain medium and necessary optics hardware including correction optics, an optical channel, and shaping optics.
An approach has been suggested that feeds light into a pumping chamber with fewer required intervening optics hardware (Friel, et al., “High average power cw face pumping of a Nd:YAG planar waveguide laser with diode bars,” IEEE Conference on Lasers and Electro-Optics Europe, September 2000). As shown in FIG. 10, a stack of diode bars 500 face pump lasers that are guided into a slotted mirror pump chamber 600. The diode bars are positioned very close to the precision array of slots 601 of very small width in the top of the pump mirror 602 to guide radiation into the pump chamber 600. The natural divergence of the diode bars leads to high uniformity. However, because of the natural divergence of the diode bars, the slotted mirror 602 is required between the diode bars and the waveguide laser.
Furthermore, in the case of an edge emitting diode the output facet of an edge emitter is fragile and cannot touch another surface. Consequently, the sensitive output face needs to be protected.